In many cables, it is customary to provide protection for a cable core, comprising a plurality of conductors, with a metallic shield. The metallic shield may be aluminum, which is used for lightning protection and protection from stray electromagnetic interferences, or it may be steel which is used to protect the cable from rodents, for example.
These shields are formed about the cable core as the cable core is advanced along a manufacturing line. Generally, the shield is formed with an overlapped longitudinal seam, which is such that the edge portions which provide the seam remain in place as the formed shield is moved with the cable core.
Afterwards, it is customary for the shielded core to be advanced through a passageway of a core tube of an extruder where a plastic jacket is provided about the shield. Generally, the passageway of the core tube of the extruder through which the shielded core is advanced is slightly larger than the outer diameter of the shield and is defined by an inner wall of the core tube.
As it is advanced through the core tube of the extruder, the shield with its overlapped seam remains in place about the cable core because of the method of forming the overlapped seam and because of the thickness of the shield material.
In the recent past, a new line of communications cables, which are referred to as electronic communications cables, have been made available to the marketplace. Generally, these kinds of cables are small pair size cables, involving, for example, only two to five plastic insulated metallic conductor pairs of twisted or untwisted conductors. Around the pairs of conductors in one such cable is placed a polyester tape for dielectric protection and a shield in the form of a laminated metallic foil. The laminated foil is relatively thin and an overlapped longitudinal seam of the shield which is formed along a manufacturing line must be maintained in place as the shielded core is advanced through the extruder which causes a plastic jacket to be tubed about the shielded core. Otherwise, the electrical characteristics of the cable may be affected adversely.
The maintenance of the shield configuration is accomplished generally by sizing the passageway of the core tube of the extruder through which the shielded core is advanced to be substantially the same size as the outer diameter of the shielding foil. As a result, the overlapped seam is maintained in place by the core tube up to a point just prior to the contact of the plastic extrudate with the foil.
This arrangement presents some problems in that the relatively cool metallic foil which is wrapped about the cable core just ahead of the core tube engages the inner diameter wall of the core tube and functions as a heat sink. As a result, the tip of free end of the core tube assumes a relatively cool temperature and some of the plastic material which comprises the extrudate begins to solidify prematurely on it. The result is a somewhat irregularly shaped jacket or jacket with unsightly defects in the outer surface thereof which is formed about the shielded cable core.
A further problem in the production of a relatively small electronic communications cable relates to the accumulation of air from the cable moving along the manufacturing line at a relatively high speed. As pressure builds up within the extruder, there is a problem in exhausting the air inasmuch as the core tube is sized so tightly about the shielding foil.
What is needed and what does not appear to be in the prior art are methods and apparatus for extruding a plastic jacket about a relatively small cable, which includes a laminated foil type of shield. Such methods and apparatus should also provide for the exhaustion of pressurized air and vapors that build up between the extrudate and the cable, notwithstanding the contiguous relationship of the foil with the inner wall of the core tube.